Measurement of relative delay of wave envelopes



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United States Patent O MEASUREMENT OF RELATIVE DELAY F WAVE ENVELOPES Kenneth W. Pfleger, Arlington, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application April 25, 1951, Serial No. 222,832

6 Claims. (Cl. 324-57) This invention relates to electrical measuring apparatus and methods and more particularly to apparatus for measuring envelope delay distortion and relative delay time in the transmission of waves or signals over an extended transmission system as a function of frequency.

ri`he invention is applicable to systems in which the sending end and receiving end are separated any given distance and does not require a return transmission path to the sending end, except as may be necessary for intercommunication between personnel at the sending and receiving terminals, which intercommunication may be carried out over any available return line, for example, by telegraph or telephone.

A feature of the invention is the sending of test signals alternately at a standard reference frequency and at a variable measuring frequency.

Another feature is automatic clocking of the interval between the arrival of successive test signals at the receiving terminal.

A further feature is a display of the relative delay time at the receiving terminal.

In the figures:

Fig. l is a single-line schematic diagram of an illustrative system embodying the invention;

Fig. 2 is a single-line schematic diagram of an illustrative sending device;

Fig. 3 is a double-line schematic diagram of an illustrative sender in greater detail than shown in Fig. 2;

Fig. 4 is a group of illustrative oscillograms of cui'- rents in various circuit branches of the systems of Figs. 2 and 3;

Fig. 5 is a single-line schematic diagram of an illustrative receiving device;

Fig. 6 is a double-line schematic diagram of an illustrative receiver in greater detail than shown in Fig. 5;

Fig. 7 is a group of illustrative oscillograms of currents in various circuit branches of the systems of Figs. 6 and 7;

Fig. 8 is a group of enlarged oscillograins of currents in certain circuit branches in the systems of Figs. 6 and 7; Fig. 9 is a schematic diagram of a noise indicator;

Fig. l0 is a tabulation of a sequence of relay operations involved in the operation of the noise indicator of Fig, 9;

Fig. ll is a schematic diagram of an alternative gating system for use ata sending terminal; n h

Fig. l2 is a schematic diagram of a gate control circuit employing a pentagrid vacuum tube; and

Fig. 13 is a schematic diagram of a portion of an alternative receiving arrangement.

General method and apparatus in accordance with the invention, the method of measuring envelope delay distortion as applied over a straightaway circuit or line, comprises (a) sending periodically onto the line to be measured a pulse of reference frequency tone and a pulse of comparison or measuring frequency tone, one after the other, preferably with restricted sidebands, (b) counting the time elapsing between arrivals of particular amplitudes of these pulses envelopes at the receiving end, and (c) averaging the results of a multiplicity of such counts when it is desired to reduce fortuitous effects such as caused by noise. The averaged count may be automatically indicated; and successive indications, each taken with a different measuring frequency and using the saine time interval between sent tones, give substantially the envelope delay distortion vs. frequency characteristic except for a constant delay which is usu- Y 2,700,133 Patented Jan. 18, 1955 ally unimportant. if the absolute delay is desired, it can be obtained when measuring is done on a loop rather than on a straightaway basis.

A variety or' methods are known for the independent transmission of the reference and variable frequency signals. Of these, one in which time division is used is preferred, since it has the advantages of requiring only one circuit and of eliminating modulation difficulties between the two tones although at the expense of some storage mechanism at the receiver.

Usually two-way communication will be desirable between persons at each end of the line under test, and can be carried on over the same line during intervals between measurements. When the line does not transmit in both directions some auxiliary form of communication, for example, a telegraph or telephone circuit, may be used to convey intelligence back and forth from sender to receiver.

Usually the narrower the range to which sidebands of the measuring (variable) frequency signal are restricted the better, as the delay measured is then more nearly the same as the theoretical value, .//clw, except that the received envelope wave is then so gradual in slope that it is very dimcult to measure time accurately to a particular point on the wave. As a compromise in a telephoto circuit, e. g., it is practical to receive a sinal spectrum about cycles wide from the line having sidebands each 25 cycles wide. The sent waves envelope is rounded, closely approximating the function c(1} cos 50a) where c is a constant and t has values from .02 to -i-.02. Outside this range of t the envelope is zero until time for the next mark. This gives rise to wide band of frequencies but the energy outside the 50 cycle band is small. Due to the cut-off of the receiving band-pass filter, the received envelope departs somewhat from that of the sent wave.

The sidebands of the reference frequency signal need not be restricted to such a narrow band as 50 cycles, because it is not necessary that its delay approximate ri/dw. Any delay value is satisfactory for the reference pulse so long as it is constant. However, the greater the restriction of band width, the easier it becomes to use measuring frequencies close to the reference frequency without erroneous operation of selective receiving circuits due to overlapping of frequency spectra of these tones. By using wide sidebands for the reference tone and by giving it a steep wave front the need for much amplification of the reference wave at the receiving end diminishes, as discussed below. Of course, the frequency spectrum of the reference tone should not contain much energy in the frequency range used for the measuring tone, and consequently it is then diicult to measure delay distortion very close to the reference frequency. The use of two reference frequencies is then generally necessary in order to obtain a complete delay distortion characteristic over all parts of the desired frequency' range. Although the illustrative examples disclosed hereinafter do not use wide sidebands, this possibility is available in case one wishes to trade cost and weight of apparatus for the slight inconvenience of using two reference frequencies.

In the event that a 5t) cycle band width is used for both the reference and measuring frequency marks, one may use measuring frequencies as close as about l0() cycles to any reference frequency. Furthermore, by using two different reference frequencies successively it is possible to obtain the delay disto tion anywhere in the pass band of the line, when the delay distortion characteristic obtained with one reference frequency is modified by a suitable constant so that one continuous characteristic results when both are plotted on the same graph.

In the counting mechanism, to measure the time interval between the arrivals of particular amplitudes of the reference and measuring tones, measuring preferably is done between peak values of the detected envelope waves of these tones because small amounts of level variation or selective attenuation on the line do not appreciably affect the mechanisms ability to discern the instants of maximum current. These instants are easily recognizable after passing the detected envelopes over a differentiating circuit because the latters output -wave passes through Zero at each maximum of the received wave, andthe rapid change in sign of the diiferentiators output is suitable for the precise operation of trigger circuits for starting or stopping an electronic counter.

In order to insure still more reliable operation of trigger circuits the transitions in the differentiators output wave may be greatly amplified, as is accomplished partly by non-linear coils which generate a sharp impulse of high voltage, and partly by linear amplification.

Separate amplifiers and differentiators may be used for the two detected signals, although some saving in cost may result from a reduction in such apparatus, such as by the use of wider sidebands for the reference frequency signal as explained above.

Although an electronic counter measures delay precisely in one observation7 the value may be influenced by interfering effects such as caused by noise. Therefore it is generally advantageous to design the counter so that it makes a number of delay measurements and indicates the average value.

Outline of illustrative system Fig. l is a simplified schematic diagram of an illustrative system having the basic elements required in the practice of the invention. Sinusoidally-enveloped pulses of a suitable high-frequency carrier are produced at accurately timed intervals at the sending end of the transducer 1 to be measured, such as a line by means of a pulse repetition frequency source 2 of frequency such as 25 cycles reacting with a carrier oscillator 3, of frequency such as 97,000 cycles, in a first modulator 4. The resulting modulated wave is impressed upon a second modulator 5 along with selected and timed pulses of standard and variable comparison frequency oscillations of uniform duration from oscillators 6 and 7 respectively. The pulses of standard frequency are under the control of a first gate 8 and those of the variable frequency under the control of a second gate 9, both gates being operated by a gate control device 10 which in turn is under the control of the low frequency source 2. The device 10, upon being manually started by the operator, first opens gate 8 to pass one pulse of standard frequency, then closes gate 8 for a selected number of pulse intervals, generally only one pulse interval, as in the illustrative system, then opens gate 9 and pass one pulse of cornparison frequency, closes gate 9 and holds both gates closed for a selected number (one or more) of pulse intervals and repeats this series of operations until shut down. The modulator 5 shifts the standard and variable comparison pulses downward in frequency to the desired values of frequency with which the transducer 1 is to be tested, for example, the standard pulses may be shifted to 3,000 cycles and the variable frequency pulses to the range 200 to 20,000 cycles, a low pass filter 34 being inserted between the modulator 5 and the transducer 1. At the receiving end of the line a band filter 11 selects the 3,000 cycle pulse which is thereupon translated into a sharp impulse by means of a detector 12, a differentiator 13 and an impulse generator 14. To handle the pulses of variable frequency efficiently it is usually preferable to shift them to a common frequency band as by means of a frequency shifter 1S controlled by a variable frequency oscillator 16. The common frequency may advantageously be placed at 97,000 cycles in the illustrative example. The 97,000 cycle resulting pulse is translated into a sharp impulse by means of a detector 17, a differentiator 1S and an impulse generator 19. The first impulse upon the generator 14 is used to actuate a start-stop mechanism 20 which in turn actuates a gate device 21 which connects a standard counting cycle generator 22 to a cycle counter 23 which counts cycles from the generator 22 as long as the gate 21 remains open. The next impulse, which comes from generator 19, shuts off the counting cycle generator, and, registers a count of one on a sample counter 24. When the last sample has been received and its number of cycles counted, the sample counter 24 stops the counting process by actuating the start-stop mechanism 20 to close the gate 21. The total count is shown on the count indicator 25, and is an automatically averaged value of all sample counts, as explained more fully hereinafter.

Sending apparatus i la at its output has only three frequencies:

oscillogram of current I'z in Fig. 4. The reference frequency as illustrated is (97 kc.-fa) where fa may have any convenient number of fixed values, and the variable frequency is (97 kc.--f=b) in the example shown. These two voice frequencies are the result of heterodyning in the example hereinafter described. The envelope wave of these spurts is substantially sinusoidal as explained below, and resembles a 25 cycle wave in the system. The duration of each spurt is .04 second when the modulating wave is 25 cycles. The time elapsing between the peak value of the reference tone and the peak value of the next following spurt of variable frequency tone is assumed to be fixed, at .08 second. Therefore there is zero output during .04 second of this interval. This interval may be made greater if desired depending upon whether the envelope delay in the line is appreciably greater at the reference frequency than at any variable frequency used, it being necessary that these pulses shall not overlap at the receiving end of the line.

The choice of the time elapsing between the peak value of the variable frequency tone and the peak of the next succeeding reference frequency spurt depends upon the amount of delay distortion to be measured, since it is necessary thatthe spurt of variable frequency tone shall not overlap or run into the spurt of reference frequency tone at the receiving end of the line. lt is evident that the duration of the zero current interval following a pulse of variable frequency at the sender output should about equl the maximum value of delay distortion to be measure Illustrative spurts or pulses are shown in Fig. 4 as current I7 and in order to obtain current I7, the sending device may be shown schematically by the single line diagram in the upper portion of Fig. 2.

In Fig. 2, a source 26 of accurate frequency provides the input wave of a multivibrator or frequency divider 27 Whose output is the first modulating frequency, for example, 25 cycles per second in the case of certain telephotograph circuits.

The 25 cycle wave passes over a hybrid coil Si? and one output thereof is connected to one input of the first modulator 4. The other input of this modulator is an accurate high frequency, for example 97 kilocycles obtained either as a harmonic of the accurate source 26 through the connection 29 as shown (or from the frequency divider 27). The value 97 kilocycles is assumed for illustrative purposes. A narrow band-pass filter J1 is shown at the output of the first modulator 4. rIlle sidebands passed by this filter are narrow so that the current 96.975, 97.0 and 97.025 kilocycles. The amplitudes of 25 cycles per second and 97 kilocycles applied to modulator 4 preferably arc adjusted so that I3 is 100 per cent modulated as shown in the oscillogram of Fig. 4. If the control source 26 is a 50 cycle per second tuning fork the connection 29 may be used, 97 kilocycles being the l940th harmonic of cycles per second. This may require several stages of harmonic generation. If the control frequency is l kilocycle,4 only the 97th harmonic is required.

The harmonic generator 28 may provide also any desired number of accurate high frequencies near 97 kilocycles such as values shown in Fig. 2 at two other outputs of the generator, and discussed more fully below. The particular high frequency desired may be selected by means of a switch S4 and is the frequency referred to hereinabove as fa. A rectangular spurt of frequency f2. is passed by the first gate 8 starting and ending when the envelope of I3 has zero value as shown in the oscillogram of le. A rectangular spurt of .04 second duration of frequency fb from the variable high frequency oscillator 7 ,is passed by the second gate 9, .04 second later than the spurt of fa as shown in the oscillogram of Is. The gates 8, 9 and the gate control 10 are described more fully below, and are synchronized by a current of 25 cycles from the other output of the hybrid coil 30. A balancing network 32 for the hybrid coil 30 is indicated schematically, and a pad 33 is shown connected between a louI pass filter 34 and the line to be tested.

The currents I3 and Is enter the second modulator 5 and the lower sideband in the output is selected by the low pass filter 34, the current I7 being obtained. When current Is has long continued zero value Iv also is zero because the current I3 does not appreciably pass over the modulator 5, which is preferably balanced for this purpose, and .thelow passilter 34fcuts'oswellibel0w 96:975 i kilocycles Fig; 3 showsaanf illustrative sender. in greaterl de'tail with 1an arrangementrof gates'fand gatefcontrol circuits using.- gas tubes and relays..v

In Fig.A 3 the sources of -fa and197 kilocyclesare shown as lprecise oscillators, though infmany-'practical applications it may be preferable to use :the'harmonic generation indicated in Fig; 2. Except for the ,individual oscillators shown in Fig.Y 3, the'operation ofthisarrangement issubstantially the-same asfor Fig.v 2, but more details are given.

The source 26 is replacedbya controlled tuning fork 3S.' which may-havefa frequency of V50 cyclesfper second with a precision of, for example, li..oneone-thousandth of one-.per cent. Thermultivibrator 27Jmayhavea frequency dividing ratioof 2:1givinganoutput of 251 cycles, which is preferably passed .through a low pass filter-36.V A pad 37 is shown between. the hybridcoil 30 and the modulator 4.! The 'harmonic generator 2S is not needed and is omitted.. Arepeatng coil`38 maybe used between the filter 31 and the modulator 5 for matching impedances. the harmonic generator 2S..in Fig. 2'is Vsuppliedinstead byan oscillator 39, which' may be crystal controlled to the same precision as the tuningfork 35.y The 94 kilocycles or 96 kilocycles output provided byl harmonic generator 25 in Fig. 2 is supplied instead byran oscillator 49 which` may also be lcrystal controlled to the same precision.

The peaked wave form of current vIs described below and shown in the oscillograms in Fig. 4 would be particularlyn useful for activatinga harmonic generator, although none is shown in Fig. 3. The ZS-cycle low pass filter 36 shown at the multivibrator outputin Fig. 3 may be desirable to insure a suitable approximately sinusoidal Wave form in the generator of Ir described below, the design of which. is based on current I4 being sinusoidal.

In Fig. 3 provision is made for azerocurrent vinterval or" any desired multiple of .04- second following-the sent variable frequency pulse in current I7. This zero current interval governs the value of N, the number of gas tubes ina series, G. T. i to G. T. N, shown at the left of Fig. 3. The operation of thesetubes is described below from which it is apparent that the greatest delay distortion which can be measured with the arrangement shown is aboutv .04 (iN-3) second and the timeelapsing between the peak value of the sentvariable frequency tone and the peak of the next succeeding sent referencefrequency` spurt is .Q4 (N-Z) second. The period-.of repetition of the sent signal is .04N second.,

While large values of N enable oneto measure great amounts of delay distortion, .itshouldbe noted that the.

time consumed in the automatic counter and averaging device is also proportional to N. In order to speed up the'testing of short lines and to provide equipment for measuring long ones, it is advisable'to make N variable.

Means for varying N are not shown in Fig. 3 but can be included by means of keys (not shown) so that N may be made to vary eiiectively from 4 to any desired larger value, the minimum number of gas tubes required in measuring small distortions generally being 4.

The upper path from the hybrid coil 30 conducts the cycle current I1 through the resistance pad 37, for the purpose of adjusting the level, to the modulator 4. The current I2 at the modulator output ,is substantially 25 cycle carrier telegraph reversals, and need not have a sinusoidal envelope. The amount of vcarrier leak from the modulator is to be adjusted so that the wave Ig as seen at the band-pass filter 31 output is 100 per cent modulated. A cathode ray oscilloscope (not shown) is desirable to check the Wave form of la and I'z.y It may be a small portable oscilloscope.

The current le at the other input 'of the modulator 5 of Fig. 3 is substantially the same as in Fig. 4 andresults from operation of gates 3 and 9 of either figure to pass rectangular spurts of fixed or variable frequency, fa or fb, respectively, each of .04 second duration, and timed so as to occur just when the'spurt of (97 kilocycles-fa) or (97 kilocycles-fs) is desired' in output current i7. (In this statement the delay of low pass tilter 34 has been* neglected as it may be very small.) Ther method for operating the gates is described below.

The filter 34 at the output ofthe :modulator Sishould be .balanced .and Vshould Jpreferably have .negligible delayj The 97 kilocycles output provided byY output of the hybrid coil 'i henries and from 2to 8 ting for this phase shifter depends .mainly'on the delayl 6. distortion 1in; the vmeasuring frequencyrange of the set, say 200:.to :20,000 cycles perisecond. If diiiculty is vexperiencediinrobtaining negligible delay-distortion, the cutoff; frequency of the filter 34 may be raised to 70 or 80 kilocycles, ormore. The purpose of the -pad 33 between the lter 34 and the line is to control the level of current entering the line or transducer to be measured, and to obtain a suitableA terminating yimpedance at the sending end thereof.

The-spurts of faand fb 'should be substantially equal in-amplitude.- In order to make this adjustment one may observe Is on the oscilloscope while adjusting the volume controls (not shown) oftheoscillators .40 and 11. Another method is toobserve the levelV at the output or" the pad 41 first Withgate Sopen and gate 9 closed and second with 'gate closedand gate 9'opened. Switches for manually controlling opening and closing -of'theser gates are provided and described below.'

The variable oscillatorf16 vprovides a continuously/'adjustable frequency over the range say 77 to 96.8 kilocycles so that the value of 97 kc.-fb appearing at theinput of the line. ranges from-200 to 20,000 cycles per second.

in order to control lthe opening and closing'of gates 8 :and 9, the accurate 25 cycle frequency from the lower 30 is passed through an adjustable phase shifter 2consisting of an'inductance L31, variable condenser C31 and a reversing switchSs.

The current I4 should pass through zero very close the Zero values of the envelope of I3, because switching isthen desired controlled by sharp pulses in current i5 producedA by-a non-linear coil L32 of Aa network L32, C32, Rs.. The requirement is `that lthe voltage generated across resistance R2 shall. build `up rapidly reachingS volts ormore .within-about 2 microseconds. If this cannot be done satisfactorilywith thecoil L32 alone, it can be done using a multistage amplifier limiterinstead, provided the gridbias does not vary. The reason forvthisrequirementisthat the switching ofthe ygates must be done at precise mstants; Lat and .C31 should have high reactances at 25 cycles per second in order to prevent the discharge from C32 from being dissipated too much in the 25 cycle source.-. The exact values .of C31 and L31 may be readily found by trial. The hybrid coil 3i) may be of inequality ratio. since only about +20 dbm. (decibels above milliwatt.) -would berequired for I1. The coil VL32 may be of the Vtype described byPeterson, Manley and Wrathall in Magnetic Generation or" a Group of Harmonics, Bell System Techanical Iournal, vol. XVI, page 437, 1937.

llustrativevalues for L31 and C31 are from 6 to l2 mf. respectively. The iinal setin the viilt'erl.l The phase shifter gives a range of nearly a quarter cycle for each position ofswitch S5. Additional phase shift range can be obtained if necessary by shuntmgreactance directly across the lower output of the hybridicoil 30.

Successive positive peaks inV current I5 generate sumcient voltage across resistance R2 to lire gas tubes G. T. 1 to G. T. N, inclusive, when plate voltage is connected. Protective resistors r1 to rN, inclusive, are shown in series with the grids soV that there is no appreciable current drawnfthereby. Itis possible that these resistors in some casesmaynot be needed. The gas tubes have their respective anode circuits operatively connected to individual relaysSRr. to SRN inclusive.

Switches S1 and S5 are normally closed and S2 and S3 are normally open. Before closing Si all relays SR1 to SRM, inclusive, .arereleased andL gas tubes not firing. The .normal .grid biases E1 and E2 of gates 8 and 9 are suiiicient tov prevent transmission of'oscillators outputs through the gates After closure of S1 the first lpositive pulse in Is causes gas tube 1 to fire and current z`1- begins, causing a direct current voltage drop across aresistor B1 which counteracts part of the negative grid bias and puts gate 8' in a transmitting state, so that frequency fa is transmitted by gate 3. The rise of i1 operates slowly the relay/SR1, (SR1 must be slow enough not to operate before .the hrst positive pulse of I5 has died out or SR2 may operate prematurely), and connects battery El to the'plate of G. T. 2 which fires on the next positive pulseof is. Then .the-current i2 begins. Thevoltage drop across a resistor B2 is equal and opposite tothat caused by i1 across B1. The result is immediate closure of gate 8 since the vgrid 'bias is 'then equal to the full value .of E1.

When z'z begins, relay SR2 operates slowly (for a similar reason to SR1) and connects battery E4 to the plate of G. T. 3 which fires on the next positive pulse of Is. The rise of current i3 opens gate 9 because of the voltage drop across a resistance B3, and slowly operates relay SRs to put plate battery on G. T. 4.

The next positive pulse of I5 causes G. T. 4 to re. The rise of 131 closes gate 9 because of the voltage drop across B4, and slowly operates relay SR4. Succeeding positive pulses of i5 operate the other gas tubes in this series one at a time in a similar manner, but their protective load resistors Bn-r, B11, BN-r, and BN do not operate any gates. When the last gas tube, G. T. N, has been fired, its plate circuit relay SRN opens Contact to extinguish all gas tubes. Relay SRN should be slightly slow in releasing so that it does not release until all gas tubes are completely extinguished. 1t is clear that the number of gas tubes in this series determines how long it takes before all are extinguished and the circuit ready to begin over again the procedure of opening and clos-v ing the two gates successively.

Thecurrents i1, i2, i3 and i4 should reach steady state value without overshoot almost instantly once the correspondingly numbered gas tube has lired. Therefore small condensers, c1, cz, c3, and c4, et cetera, are shunted around the respective relay windings. This also tends to slow down the relays operation as desired. Values of these condensers are readily found by trial after observing the gas tube currents with an oscilloscope.

When high values are used for resistors B1, Bz, B3, BN, the currents i1, i2, i3 in may be small and the gas tubes may be miniature. On the other hand these resistors should not be too high or vacuum tubes V. T. 1 and V. T. 2 of gates 8 and 9 respectively, will have their cathodes far removed from ground and might become noisy. Suitable values are readily found by trial. The oscillators of frequencies fa and fb may be connected either directly to the grid bias battery as in the case of oscillator 49, or directly to the grid as in the case of oscillator 16.' Trial should readily reveal which location is preferable from the noise standpoint, and then both oscillators can be connected in the preferred manner to their respective gates.

A spark killer 43 may be connected across contacts of relay SRN to reduce contact deterioration, but should be designed so that the gas tubes go out quickly. Otherwise spurious gate openings may occur if the decays of the plate currents are unequal.

On opening S5 Vand S1 all the relays release. Then one may close S1 and Sz if one desires gate 8 to be open for a longer time. By closing S1 and S3 gate 9 is made open. On opening first one gate and then the other in this manner one may observe the effect on level in le or lr due to adjusting the power outputs of osciliators of frequencies fa or fr, as previously mentioned. Resistors Ln and Lo may be provided to give the same resistance as a gas tube when tiring.

The advantage of the electronic gates is that commutation occurs very Vrapidly and without the usual troubles found in mechanical distributors such as brush chatter or variation in contact resistance.

The filter 31 attenuates greatly any sideband frequencies in I3 which are more than 25 cycles from 97 kilocycles.

One may tolerate some minor departure in the loss characteristic of the filter 31 from perfect symmetry about 97 kilocycles as well as some small inaccuracy in the setting of the phase shifter 42, so long as the spurts of tone of reference and Variable frequency in I7 have the same well rounded envelope shape, which should be satisfactory for measuring envelope delay even if not perfectly sinusoidal.

Direct current sources E1, E2, E3, E4, E5 and Es are shown as batteries. Obviously one may use another form of direct current potential source but the requirement is that certain of them should be entirely separate. Thus if E3 and El have their common side grounded, no ground should be on the other direct current sources.

if trouble with noise in V. T. 1 and V. T. 2 is experienced, a separate heater supply may be necessary so that they are not heated from the same source as the gas tubes.

Receiving apparatus 1n order to carry out the function of the receiving titl device as stated above, the apparatus shown by the single line schematic of Fig. 5 may be provided.

The current Is shown in the oscillograms of Fig. 7, received from the line closely resembles I7 of Fig. 4, except for such difference between arrival of spurts of reference frequency (97 kilocycles-fa) and variable frequency (97 kilocycles-fb) as is caused by envelope delay distortion of the line.

The current Ia after traversing a pad 44 divides at a hybrid coil 45 with balancing network 46 into two paths. The reference frequency passes through a buffer amplifier 47 and the band-pass filter 11 and enters detector 12 (DET. 1) through a buffer amplifier 59. The carrier frequency is eliminated by a low pass filter 48 (L. P. F. 1). The resulting direct current wave I9 is shown in the oscillograms in Fig. 7 as a rounded E signal. It is -not quite sinusoidal due to band width limitation in the receiving narrow band-pass filter 11. Both reference and variable frequency spurts from the upper output of hybrid coil 45 enter a signal shifter comprising a modulater 49 (MOD), a variable oscillator 16 and a 97 kilocycle band-pass filter 50 (B. P. F. 3). The variable oscillator is 'set to give the frequency fb so that pulses of frequencies (97 l\'iloeycles-fb)V on entering the modu- Vlatot 49 are shifted to 97 kilocycles and pass through B. P. F. 3. The 97 kilocycle output of a harmonic generator 5l further mentioned below and a cathode ray oscilloscope 52 (C. R. O.) are useful in checking the frequency of the shifters output. Pulses of frequency (97 kilocycles-a) are shifted by the modulator 49 but then lie outside the pass band of B. P. F. 3. The output of this band-pass filter is next amplified in amplifier 53 and detected in detector 17 (DET. 2). The carrier frequency is eliminated by a low pass filter 54 (L. P. F. 2) and the current 111 is obtained similar to I9 except for a delay.

Both i9 and l11 are next dierentiated in differentiators 13 and 18 respectively to yield waves 11o and 112. These waves pass through zero at instants corresponding to maxima of ls and 111 respectively.

The waves 11o and 111 act upon impulse generators 14 and 19 respectively, which preferably employ non-linear coils to produce the impulses 151 and isz respectively having sharp positive peaks building up at a fast rate so that prompt and definite action of the connected circuits results. These circuits comprise a gate control circuit 55, the gate 21, a megacycle counter S6 having a plurality of binary stages, and a set of decimal counters 57 with indicators.

Once a start button (S11 to be described hereinafter with reference to Fig. 6) has been pressed which automatically resets all counters on zero, the positive pulses of 151 open the gate 21 and the positive pulses of 152 close the gate. A megacycle current 11s, produced by the harmonic generator 51 activated by periodically repeated pulses of 151, ows through the gate 21 during an interval of time T equal to the interval between the occurence of a positive pulse in 151 and the next succeeding positive pulse in isa.

Each time a pair of these pulses occurs a spurt of megacycle current of duration T enters the binary megacycle counter 56. This counter and the connected decimal counter 57 operate as described below to indicate the average value of T in microseconds on a digital register, which may be luminous and may show six digits, more or less, as desired, or may be arranged through known printing mechanisms to record figures, or upon a paper tape.

Sample counter 2d sends a negative pulse into the start and stop mechanism 2G which stops all counting when sucient repetitions of the interval T have been measured and averaged, as described below.

ln prior art systems employing a dynarnometer, the operator of the delay measuring set can teli when excessive noise is present by unsteadiness of the dynamorneter reading as explained. ln the case of random noise he must watch the dynamorneter deflection a while and estimate an average reading. ln the case of single frequency noise he can change the variable carrier frequency slightly and eliminate the eilect. in the set herein described, the presence of noise may vary the location of peak values of I9 and 111 which tends to make unequal the successive intervals of megacycle current such as T. This inequality can be detected by a noise indicator 53 which operates under control of the gate control circuit as described below. The meter of the noise indicator may be calibrated to indicate the number of repetitions of the interval T which ought to be averaged in the case of random noise. Then the operator can see how to set switches in the binary counters which govern the number of repetitions of the measurement of T as described hereinafter. The presence of a single frequency noise usually causes the noise indicator meter to deiiect. its effect should be eliminated by varying the carrier frequency until the noise does not pass the narrow band receiving filter as discussed more fully below.

1n Fig. 6, the harmonic generator of Fig. 5 is replaced by separate precise oscillators which need not be used in practice if the harmonic generator is cheaper, lighter or otherwise preferable.

The pad ed is provided for coarse adjustment of received current, since it is desirable to maintain fairly constant levels into the modulator 49 and the amplier 57. Pad ddl also provides a suitable terminating impedance for the transducer under test. Very constanty levels should be maintained in the two detectors as by means of slide-wire potentiometers (not shown) located within power amnl'ers 59 and S3, and a pair of sensitive and sluggish direct current level meters 60 and 6l may be held by 1hese manual controls to constant deflections in order to reduce phase variations caused by non-linearity in the detectors l2 and 17. The former detects the reference frequency tone while the latter detects the variable frequency tone after it is shifted in frequency to 97 kilocycles by the variable frequency oscillator and modulator in the previously discussed manner. The detected pulses 1s and 111 are also shown qualitatively in the sketch of oscillograms on 7 and resemble the modulating wave form described above except for slight distortion due to line and lters which is unimportant because it is practically unchanged over the range of testing frequencies.

The narrow band-pass filter 50 of approximately 50 cycle width is used in the signal shifter to reduce the line noise and other disturbances if present.

The band-pass filters 62 and 63 for two reference frequencies are also of about t) cycle width when the sent pulses of reference frequency are thus restricted in frequency pe rurn as this favors measuring fairly close to the reference frequency without appreciable error.

The low-pass filters 48 and 54 immediately following the respective detectors 12 and 17' consist of series resistors and shunt condensers because inductances at high levels are apt to be non-linear and to cause varying effects as the level changes slightly. These filters suppress the carriers. No wave shaping is required of these filters due to the wave shaping action of the sending band-pass lters.

Currents 10 and 112 are substantially proportional to the slopes of currents 19 and 111 respectively due to the differentiating effect of series condensers C11 and C12, which respectively perform the functions of the differentiators 13 and 13 of Fig. 5.

Coils L21 and L22 are nonlinear and operate similarly to L2 of Fig. 3, to produce sharp wave fronts in currents 151 and 152 so that the voltages across resistors R21 and R22 rise from O to 8 volts in about 2 microseconds. The optimum values for circuit elements connecting to these nonlinear coils may be found by trial.

Inductances L11 and L12 help to suppress the carriers and prevent discharge of condensers C21 and C22 toward the source. These inductances should be substantially lin ear. Each may consist of one or more retard coils. The inductance of such coils should be determined experimentally to obtain Vthe desired maximum amplitudes for 11o and 112 with a minimum of input power and without excessive transients. initial fluctuations during the build up of 11o and 112 are not wanted as they may cause unwanted pulses in 151 and 152 ahead of the desired times which might give false operation of gas tubes.

A zero reset switch S11 is provided and when this has been closed momentarily, gas tube G. T. 11 is not lir- 7 ing and neutral relay NR4 is released so that a pulse of 151 is transmitted over the contact of NR4 and resistor r21 to the grid of gas tube G. T. 21 which iires to open the vacuum tube gate 3S in the manner previously described in connection with gates in Fig. 3.

The next succeeding pulse in 152 is transmitted over resistor .rs1 to the grid or" gas tube G. T. 31 which fires to close the gate. Accordingly the megacycle current 113 which passes the gate has the duration T1. This is equal to the time interval elapsing between the sharp rises of 10 the respective pulses in 151 and 152 provided currents :'21 and isi rises abruptly with equal slopes to full value when the respective gas tubes re.

Resistors 1'21 and 1'31 should be r1 to rN of Fig. 3.

A relay 8R31 is necessary in series with the plate of gas tube G. T. 31 which operates to cut oit its plate voltage from source E11 and extinguishes both gas tubes very soon after gate closure.

A relay, 8R21, having substantially identical winding impedance to that of 8R31, is provided in the plate circuit of gas tube G. T. 21 so that both plate circuits have substantially identical transient responses. The windings of 3R21 and 8R31 may be shunted by small condensers so as to obtain the nearest approach to rectangular on and off switching of the gate. The contacts of relay 8R21 are open at the firing instant of gas tube G. T. 21 to make sure that tube G. T. 21 tires before tube G. T. 31, but close shortly thereafter to permit tiring of gas tube G. T. 31 on the next pulse of lss.

The megacycle pulses in 11s are shown again to an enlarged scale in Fig. 8. They may be passed through a wave shaper 65 to produce negative pulses of the saine fundamental periodicity, as shown in the oscillogram of 114. Such pulses are suitable for operating binary counters C. 21, B. C. 22, et cetera. In some cases, the Shaper 65 is not needed, the binary counter operating satisfactorily without the shaper. The Shaper may be made of either tube circuits or varistor circuits, or a copper oxide varistor with a direct current bias may be used to pass short pulses of only one polarity.

The counting mechanism, described below keeps adding up the number of megacycle pulses it receives not only during time interval T1 but also during T2 and succeeding time intervals measured until a predetermined number of such intervals have passed whereupon a pulse is received by the grid of gas tube G. T. 11 from an auxiliary series of binary counters, B. C. 11, B. C. 12, et cetera, which count the number of positive spikes in 151. This pulse fires the gas tube G. T. 11 and its plate current op crates the relay NRA. to stop further spikes in 151 from acting on the counters, at which time the counter indication becomes stationary and may be read and recorded by the operator. Operation of NRI; should not occur until at least .O4 (N-l) second has elapsed following the final spike in 151, and must be complete before .04N second has elapsed, in order to allow the gate to remain open for the longest necessary value of T and to malte sure it does not open after the last value of T. Therefore relay N121 should be slow acting and its slowness should be adjuso able if N is varied.

l The restoring of all counter circuits to the zero condition is accomplished by pressing the reset key S11 which operates reset relays 66 to make suitable voltage changes nthe power supply of the counter stages, as discussed eiow.

The circuits of relays NRil, NR21, and NRzl are provided for the purpose of driving a noise indicator as described below. This indicator detects variations in lengths of successive spurts of the current 113. When these variations are excessive, the number of samples of the time intervals T1, T2, T3, et cetera, averaged by the megacycle counter should be increased as described more fully below.

The number of samples averaged is controlled by a set of switches S32, Sei, S128, and S256. When all these switches are closed to the left, 512 samples are averaged. When one of these switches is closed to the right and the others are closed to the left, the number of samples averaged is the same as the subscript on the desienation of the switch which is closed to the right.

The time which is required for the counter to average a given number of samples is about equal to that number multiplied by .04N second.

A wave shaper 67 is also shown for the current pulse used to drive the auxiliary counter. Pulses in 151 when inverted may be suitable for driving this counter. Therefore the wave shaper 67 may be nothing but an inverter, for the purpose of changing positive pulses into negative pulses similar to 11i..

1n case the sent reference tone is given wider sidebands, band-pass iilters 62 and 63 should be just wide enough to pass them. A tenfold increase in these sidebands permits the received reference signal to build up about ten times as fast and changes the shape of I9 from designed similarly to sinusoidal to nearly rectangular. The differentiated current 110 then consists of positive and negative spikes. By making use of one or the other of these spikes to iire G. T. 21 and to operate the auxiliary counter, one can omit L11, L21 and condenser C21 and probably decrease the size of amplifier 59 and detector 12. The values for R21 and for the low pass lter 48 would be changed, the resistors beingmuch smaller as they need not dissipate as much heat.

Electronic counters trigger circuits, gates, et cetera, have been used for other purposes. Consequently no detailed explanation of these elements is considered necessary. Electronic counters are described by C. B. Leslie, Megacycle Stepping Counter, Proceedings Institute Radio Engineers, volume 36, No. 8, August, 1948, pages 1030 to 1034.

Counting mechanism It is assumed in the system illustrated that during a time interval T ranging in duration from about 40,000 to 40,000 (N-l) microseconds, a precisely one megacycle current ows from the oscillator 22, Fig. 1, suitable for operating an electronic pulse counter. The time interval and the one megacycle current ow are repeated periodically, each such interval being separated by a short time of zero current. Due to small amounts of line noise not under control, slight fortuitous variations exist in the value T from one repetition to the next. In order to reduce the effect of these, and to obtain a better value of T, it is desirable to add and average a number of separate measurements of T. in accordance with the invention, apparatus is provided to automatically indicate the result in microseconds.

Let 2n denote the approximate number of repetitions of T which should be averaged in order to reduce the fortuitous effects to a negligible amount, the value for n being calculable for any small amount of noise as explained below. The megacycle pulses of 114 in Fig. 6 are made to operate first on a binary electronic counter comprising n pairs of (or n double) tubes. The output of the binary counter is connected to a six stage decimal counter. The latter counter regiscters a digit about once in each 2n microseconds. The iinal reading of the decimal counter is the desired average value of T.

Averaging of a plurality of measurements of T is eifected automatically by adjustment of a scale factor associated with the counting mechanism. An average being by definition the sum of m readings divided by rn and the sum is equal to m times the average. For example, if the sum of eight readings is 1,600,000, the average is 200,000. Hence, if the counter counts once for every eight cycles of the megacycle current, the result of impressing 1,600,000 cycles upon the counter is an indicated count of 200,000 which gives the average directly expressed. Also, if there are a total of 6,400,000 cycles impressed upon the counter in the course of 32 readings, average reading again 200,000, the counter may be arranged to count once for every 32 cycles, giving an indicated count of 200,000, thus once more directly indicating the average. By using such a scheme, the average value as indicated is independent of the number of samples averaged.

It is evident that when n= there are 32 samples of T averaged and since samples of T occur every .2 second when N=5 the running time is 6.4 seconds. It has been determined by computation that in the case of a telephotograph line on which there is a normal or average amount of random noise here assumed to be like thermal noise, the measuring error represented by the average of 32 values of T has a standard deviation of 8.2 microseconds, which should usually be satisfactory precision in measuring telephotograph circuits. Such a normal or average amount of thermal noise is sometimes referred to as a thermal noise level of 35 dba. with telephoto Weighting at the zero level point of a telephotograph line. The unit used in expressing the noise level is adjusted decibels, abbreviated dba., and refers to the reading of a noise measuring set such as the Western Electric Companys No. 2 type noise measuring set described in Bell Laboratories Record, March, 1936, volume 14, page 233, when used with an appropriate equalizer designed to weigh the disturbing effect of noise currents of different frequencies and durations upon a telephotograph circuit, the noise measurement being made at the zero level point of the line.

. Experience with telephotograph operation has indicated that the highest tolerable limit of thermal noise is reached at 45 dba. or l0 decibels above the normal or average condition. Whenl the noise is at the highest tolerable limit, the number of values of T which must be averaged to obtain the same standard deviation of error is ten times as great, or 320. In the binary scale the lowest sufficient value of n is 9, requiring the averaging of 512 values of T, since with n equal to 8, only 256 values of T would be averaged. Accordingly the binary counter needs to have 9 double tubes or stages such as B. C. 21 to B. C. 29 in Fig. 6, so that n may be varied by desired amounts between 5 and 9, using switches S32, S64, S128, and S256 as described above.

Noise indicator Noise on the line may reduce the accuracy of the timing of an interval by distorting, lengthening, or shortening of the pulses of Ia of Fig. 7, which in turn affects the shape of the detected pulses, Is and 111, and displaces the zero points of 11o and 112, and hence affects the spacing between the impulses 151 and 152. The net result is that either too many or too few cycles of the standard frequency are included in the intervals T1, T2, et Cetera, of 113. The accuracy in the presence of line noise may be improved by including a large number of intervals in the cycle count. The more the eifect of line noise, the more intervals should be included. As the effect of line noise is random, it is very likely to make itself evident by making adjacent intervals unequal. The more noise, the more inequality is likely to be found.

Although one may compute the number of values of T which should be averaged when the line noise has a certain value, it is advantageous to have a meter which indicates this automatically.

In Fig. 6 it is shown that the output of the vacuum tube gate 38 is tapped by a branch circuit delivering megacycle power to the noise indicator 58 shown simply by a box in this figure but shown more in detail in Fig. 9. Therein leads 68 and a step-up transformer 69 impress a high megacycle voltage upon a rectiiier 70 which may charge either condenser C71 or C72 over a high resistance R71. Therefore the charge on either condenser is proportional substantially to the charging time which is the same as the duration of a spurt of megacycle current such as T1 or T2, et cetera.

As shown by the table of Fig. l0, a neutral relay NRt has closed contacts during time interval T1, correspond ing to the spurt of megacycle current T1 in the oscillogram of Fig. 7. The'condenser C71 is charged during this time T1. The contacts of two other relays NH2 and NRs are then open as shown in the table. Shortly thereafter the contacts of NR1 open and this is followed by closure of contacts of relay NR2 so that during time interval T2 the condenser C72 is charged. After the end of interval T2 the contacts of NR2 open. Shortly after this the contacts of relay NRs close which permits both condensers to discharge simultaneously to ground through a pair of rectiers 71 and 72 and resistors R72, R73 and R74. The resistance R74 is adjusted so that the alternating current meter reads zero when the two condenser discharges are equal as when noise is absent. Then subsequent inequality of duration of two adjacent spurts of megacycle current will cause unequal charges upon condensers C71 and C72, and during discharge will produce meter torque about 1n proportion to the dierence between the durations of T1 and T2. The meter should be sluggish as of the thermocouple type and may be calibrated in terms of the exponent n required to satisfactorily average out the effect of the noise. (It would also be possible to operate switches S32, S64, S128, and S256 automatically from the meter with the help of marginal relays, not shown.) Condenser discharge cannot occur until relay NR; closes contacts because neither tube 71 nor 72 will conduct current `rom filament to plate. The tubes 70, 71, 72 are served by a filament supply transformer '74.

In order to insure closing or opening of contacts of relays NRr, NRz, and NRS to completely include all of the respective time intervals indicated in Fig. l0, Various means can be used to activate the relay windings. One method. is next described which uses three auxiliary neutral relays NRil, NR21, and NRal shown in Fig. 6.

Before the power supply E11 in Fig. 6 is turned on, relays NR11, NRz-, and NR3l are not operated and all their moving tongues are in the down position. After E11 is functlonlng,l thel operator starts the noise indicator by momentarily closing a start switch S12 which permits current to iiow from E11 over a tongue e2 of relay NRz1 through the winding of relay NRil to switch S12 and there to ground. This relay then operates and current continues to tiow in its winding to ground by way of tongue a1 after the start switch is no longer closed.

The operator may next press the Zero reset switch S11 and the counting mechanism starts. During the entire rst pulse of megacycle current in the interval T1 current 1'11 flows through relay NR11 as shown in the oscillograrns, and consequently relay NR1 is also operated due to closure of contact by tongue d1 of relay NRil and a connection 7S indicated in Figs. 6 and 9.

Shortly after the interval Ti, relay 8R31 operates to ground its lower tongue momentarily whereuon current 1'22 ows from source E11 over tongue es of relay NRa1 through the winding or' relay NR21, tongue b2, tongue c1, tongue of relay SRgi and to ground. Accordingly relay NRzl operates and remains operated after release of 3R31 because ot current :'22 now flowing over tongue a2 to ground. During operation of relay NR217 tongue n2 makes contact before tongue b2 breaks contact. The make before break feature can be obtained by suitable mechanical design. Gperation of NH21 opens the contact ot tongue e2 so that relay NRil releases and in falls to zero as shown in the oscillograms of F ig. 7.

At about the same time as relay NRil releases, relay NR1 releases as relays NRt, NR2, and NRS are all made fast release relays. However they are made slightly slow to operate so that closure of Contact does not occur until after the previously operated relay in this group has released. Slow operation may be attained when necessary by inserting a separate inductance (not shown) in series with the winding f each relay to be slowed.

Operation of relay NR21 ermits current to tlow over relay NR2 to ground by way of tongue d2, and a connection 7d shown in Figs. 6 and 9 and shortly thereafter relay NRL) operates. The timing is such that it operates after NRi has released and before the start of time interval T2.

Shortly after time interval T2 another brief closure of the lower contact on relay 8R31 occurs. This time cur* rent isa flows from source E11 over tongue e1, winding of NRsl, tongue b3, tongue c2, to lower tongue of relay SRsi, andy thence to ground.

Immediately relay Nlsl operates and continues to remain operated due to current 1'33 owing over tongue as after release of SRsi. (Tongue as makes contact just before tongue bs opens contact.)

Operation of relay NRzl stops current 1'22 due to opening of the contact of tongue es. Due to operation of tongue d3 and current through a connection '77 the opera tion of relay NRs takes place shortly after NR2 releases caused by opening of contact by tongue d2.

The next closure of relay 8R31 permits the rise of in again by way of tongue e2, winding of NRi, tongue b1, tongue c3, and lower contact of SRsi. After SR31 releases, iii continues by way of tongue a1 and isa is stopped by opening of contact e1. (Tongue :n makes contact just before tongue b1 breaks contact.) This concludes one complete cycle of events in the noise indicator auxiliary relays, and again places condenser C1 in condition for charging just as soon as relays NRi operate due to closure of tongue di.

Spark killers K1, K2, K3 K11 are shown to reduce relay Contact erosion. Condensers and resistances should be adequate to quench the sparks but should not slow down the current decay too lmuc Alternative gating systems for sending terminal An electric counting circuit shown in Fig. l1 and comprising a plurality of vacuum tubes and varistors may be used in place of the gas tubes and relays of Fig. 3 to reduce weight and cost of maintaining relay contacts. The oscillograms in Fig. 4 apply to Fig. 1l as regards currents shown in the latter, except for the insertion of a one-way device 78 so that only the negative pulses of current i are received by the gate control device of Fig. ll.

An N-digit counter 79 of any suitable known kind may be used. The operation of the counter is such that during the iirst period of .'04 second corresponding to a desired rectangular spurt of fixed high frequency fa in the current Is, the potential on output lead 81 of the counter 79 is positive with respect to that on lead C. This causes gate 8 'to Iopen because direct current passes over its varistors in the forward direction. At other times the potential of lead 81 is less than that of lead C and gate 8 is closed. During the third period of .04 second correspending to a desired rectangular spurt of variable high frequency fb, gate 9 is similarly operated, but at other times it is closed. Gate 8 may contain a plurality of varistors 86 and a resistor Rss and gate 9 may contain a plurality oi varistors 87 and a resistor Rat as shown.

Values for resistors Rsi and Ras are to be chosen by trial to obtain a high ratio between amplitude of gate output when open as compared to the closed condition, 35 decibels or more usually being satisfactory. When necessary to obtain a higher ratio, a multiplicity of such gates may be used in tandem, controlled by the same gate control output leads.

Switches may be provided by means of which one can open leads S1 and 33 and insert positive direct potential upon either gate for the purpose of obtaining a continuous current of either frequency fa or fr.. This is useful when adjusting the sending levels.

Some care will have to be used in order not to render the counting mechanism inoperative due to the gate varistors having too low an impedance in the forward direction. if trouble is experienced in this respect one may use a pentagrid gate so that the control lead goes to a high impedance grid, as shown in Fig. l2.

Fig. l2 shows the alternative form of gate using a pentagrid tube 83. Lead 8l or 83, as the case may be, from the counter 79 is connected to grid 103 of tube SS and lead C is connected to grid MBS. The input from either source 4d or oscillator 1i is connected through a transformer to the grid itil and the output is taken from the anode. Grids 192 and HB4 are connected together and function as screens between grids 101 and 103, and 103 and IGS, respectively. An lS-volt biasing battery 200 shown in series with lead C keeps the gate closed during times when lead S1 or 83 has no positive potential. This biasing battery could also be in series with lead 31 or S3 instead of lead C, or could be partly in each lead so long as it is poled to make grid itl more negative than grid lOS.

Alternative receiving arrangement Fig. 13 shows a portion of a receiver similar to the arrangement of Fig. 6 except that all relays but those in the reset device are eliminated. The latter relays could also be eliminated if one would put their contact leads instead on a multiple contact switch including contacts of Sn. From the line to the ditierentiators the system is the same as that of Fig. 6. Also, the megacycle counter 23 and indicator and the auxiliary binary counter 24 are respectively the saine in Pigs. 6 and la. The impulse generators i4 and 19 are the same in both figures except that in Fig. 13 negative pulses instead of positive pulses appear at their outputs, due to a reversal in the leads at their inputs, since negative pulses are required for `operation of certain known types of binary counter. Counters 24, 39 and H5 are shown as of such type, While counter 23 is shown as operatable on positive pulses, or when lit. consists of negative current as in Fig. 8, counter 23 operates on negative pulses.

The desired negative voltage pulses E51 and E52 have shapes similar to the current pulses "L31 and [52 shown in oscillograms in Fig. 7 except for the change in sign.

Vacuum tubes lilland R51 shown do not have continuous plate supply sources but are operable momentarily by negative pulses of suitable strength as in E51 or E52, under the further control of their respective grids, so that either tube is disabled when its grid is suiiiciently negative with respect to its cathode.

When the power is tirst turned on, gas tube G. T. 111 will not be tiring, but it tires when a suihcient number of pulses of E52 have acted on the auxiliary counter to produce stop pulse at its output which acts on the grid of G. T. 111 through the associated input transformer. Counters 23 and 115 stop when G. T. lil tires as explained below.

Before a delay measurement is made, one must press key S11 to operate the reset device which places all counters in the zero condition in the manner hereinabove described. It is necessary that all counters shall he reset to the zero condition before pulses of E51 or E52 are allowed to reach them. Therefore, key S11 is designed to make contact to the fast acting reset relays before plate current of tube G. T. 11.1 is vcut oli by the key, in order 15 thatall counters shall reach the Zero condition before the plate current is cut off, which has the effect of removing negative grid bias from vacuum tube 114 and opening this vacuum tube gate to permit the next negative pulse of E51 to act on counter 115.

ln case a pulse of E52 occurs ahead of said pulse in E51 and after opening of vacuum tube gate 114, that pulse of E52 can have no eect on B. C. 115 because with B. C. 115 on zero the potential of its output lead l is less than that of its output lead which biases the grid of vacuum tube 151 negatively by about 22.5 volts and prevents plate current from flowing therein due to the pulses of E52. Thus, it is evident that no pulse of E52 is received by any counter until the first pulse of E51 has acted on B. C. 115 to operate this counter to the l condition so that lead l at its output is about 22.5 volts more positive than lead 0 which opens the gate consisting of V. T. 151. Resistors R85 and Rar may be provided if necessary to limit current iiowing from leads 0 and l so that changes in potential on these leads shall not affect the performance of counter 115 through feedback action.

Gate 21 may be of either of the types shown in Figs. ll and 12, and opens promptly when G. C. 11S is in the l condition, to permit megacycle current lis to iiow into the wave shaper 65 when the first pulse of E51 occurs after operation of the start button Sii.

The next pulse of E52 may pass through the anodecathode circuit of V. T. 151 because the negative grid bias has been removed.

As soon as the negative pulse E52 reaches counter 115, the counter operates to close gate 21 and terminates the duration of the spurt of megacycle current. Succeeding pulses of E51 and E52 also respectively start and stop the megacycle current Iig in the same manner. The result is the same as in Fig. 6 in that the starting and stopping of the megacycle current occur respectively as the impulses in E51 and E52 build up.

The purpose of varsitor Vs is to prevent pulses of E51 from operating the auxiliary counter 24 or the ternary counter` 115 described below in connection with the noise indicator. V aristor Vas is provided to cut off the output impedance of impulse generator No. l during a pulse of E52 so that the full effect of the pulse acts on the counters.

ln order to insure uniform action, the wave fronts of E51 and E52 should be steep and the counter 115 should be made fast acting the same as the first stage of the megacycle counter 23 hereinabove described.

When the desired number of repetitions of the megacycle spurt has been obtained, the auxiliary counter has received a corresponding number of pulses of E52 and its last stage then sends out a pulse which fires gas tube G. T. 111 as in the case of Fig. 6. In Fig. 13 the firing of G. T. 111 causes a voltage drop across resistor R85 which biases negatively the grid of vacuum tube 114 and prevents further operation of counters 115 and 23 inclusive due to succeeding pulses until after the operator has again pressed key Sii.

Whenever a pulse of E52 passes through vacuum tube 151 to operate binary counter 115 from the l to the 0 condition, the same pulse operates the auxiliary counter 24 with a signal similar to 115 of Fig. 7 except that it occurs at the end of the time intervals Ti, T2, et cetera. Also, the same pulse operates a second auxiliary counter shown in Fig. 13 as ternary counter 89 for use in controlling gates 91, 92 and 93 of the noise indicator. These gates replace relays respectively designated as NRi, NR2, and NH3 in Fig. 9.

The gates 91, 92 and 93 are opened one at a time in the order mentioned for the duration of .04N second after the respective pulse of E52. N has the same mean- '.ig as in the previous description.

When gate 91 has opened, the megacycle current fiowing through gate 21 after the next pulse of E51, can also flow over gate 91, amplifier A4 and diode rectifier 94 to charge mica condenser Cgi through a high resistance R94 so that the final charge on C91 when gate 91 closes on the next pulse of E52, is proportional to the duration of. the megacycle spurt, for example, Ti, in Fig.' 7.

As soon as the ternary counter 89 closes gate 91, it opens gate 92, and during the next spurt of megacycle current, mica condenser C95 is charged in a similar inanner to Csi, until its iial charge is proportional to T2.

As soon as gate 92 has closed, the ternary counter opens gate 93 so that both condensers C94 and C95 discharge simultaneously over one-way valves 96 and 97, respectively, and over resistors Rss, Rsi, Rss and the alternating current meter 98. As described hereinabove the discharge circuit is balanced so that there is no meter indication when times T1 and T2 are equal. Since this noise indicator circuit will vary somewhat from time to time due to temperature changes in the condensers and resistances, et cetera, it should occasionally be readjusted for balance. ince it is not always possible to obtain a perfectly quiet line for use between the sender and receiver when making such adjustment, an inexpensive low frequency iocal oscillator should be provided in the receiver to provide signals with no uctuation noise.

The plate supply for gas tube 111 as well as its grid bias battery (if used), are non-grounded in order not to interfere with the operation of the counters, all of which operate on a separate single plate supply which is assumed to have its negative terminal grounded.

As in the case of the meter 73 in Fig. 9, the meter 9S may be calibrated preferably in terms of the exponent which determines the number of samples which should be counted to reduce to a desired degree the probable error of the measured delay time. It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

While in the sample-counting stages and in the preliminary standard wave cycle stages, binary counters are shown and are desirable in order that the number of sampies may be readily variable in powers of two, it is nevertheless feasible to use ternary counters or counters based upon other numbers. Also, wherever decimal counters are shown, counters using systems of numeration with other bases may be used instead. Where the number of events to be counted is fixed, as in the noise measuring circuit of Fig. 13, there may be but one advantageous choice of the base of the system of numeration, for which reason the use of the ternary counter 89 may be preferred for controlling the three essential steps in the noise measuring operation.

What is claimed is:

l. A system for measuring relative delay of wave envelopes in transmission at different frequencies over a given path, comprising means for sending pulses of two frequencies to be compared alternately over the given path with a known interval between the starting time of successive pulses, means at the far end of said path for determining the instants of arrival of the successive pulses, and means to measure the elapsed time between the said instants of arrival.

2. A system for measuring relative delay of Wave envelopes in transmission at different frequencies over a given line, comprising a source of waves of standard frequency, means for sending pulses of two test frequencies to be compared alternately over the given line with a known interval between the starting time of successive pulses, means at the receiving end of the line for determining a distinctive portion of each pulse to represent the instant of arrival of the respective pulse, and means to count cycles of waves of standard frequency as a measure of the elapsed time between the instants of arrival of successive pulses.

3. A system for measuring relative delay of wave envelopes in transmission at different frequencies over a given line, comprising a source of modulating potentials of substantially sinusoidal wave shape, means controlled by said modulating potentials for sending alternately over the given line pulses of two test frequencies to be compared with a known interval between the starting times of successive pulses, differentiating means at the far end of said line for determining a distinctive portion of each received puise to represent the instant of arrival of the respective pulse, a source of waves of standard frequency, and means to count the number of cycles of waves of standard frequency occurring during the time interval between the instants of arrival of successive pulses.

4. Measuring apparatus comprising means for producing reference frequency pulses, means for producing cornparison frequency pulses, control means controlled to a desired degree of accuracy for determining the time interval between successive pulses, means for alternately applying to a device to be tested pulses of reference fre- S5 quency and pulses of comparison frequency following each other at accurately determined time intervals, and means at the output of the device to be tested to measure the time interval between the arrival time at the said output of a pair of pulses comprising a pulse of reference frequency and a pulse of comparison frequency, whereby the relative envelope delay time for transmission through said device to be tested is determined for the reference and comparison frequencies.

5. Measuring apparatus comprising a source of carrier waves, a frequency stabilized low frequency source of modulating potentials, a first modulator controlled jointly by said carrier source and by said modulating potentials to produce accurately spaced carrier pulses, a

reference frequency source, a comparisonfrequency source, a second modulator, gating means controlled by said low frequency modulating potentials for impressing alternately upon said second modulator waves of said reference frequency and of said comparison frequency whereby reference pulses and comparison pulses accurately spaced are impressed alternately upon the input of a device to be tested, and means at the output of the device to be tested for measuring the time interval between the arrival at the said output of a pair of pulses comprising a reference pulse and a comparison pulse, whereby the relative pulse delay time through the device to be tested is determined for the reference and comparison frequencies.

6. Measuring apparatus comprising a source of ref- @rence frequency waves, a source of comparison frequency waves, a source of pulse forming potentials of a repetition frequency relatively low with respect to said reference and comparison waves and controlled as to frequency to a desired degree of accuracy, means controlled by said source of pulse forming potentials to produce pulses of reference frequency and of comparison frequency respectively occurring alternately at accurately spaced time intervals, means to send said pulses over a line to be tested, and means to measure the time interval between the arrival time of a pair of pulses comprising a pulse of reference frequency and a pulse of comparison frequency, whereby the relative envelope delay time of the line for said pulses is determined for the reference and comparison frequencies,

References Cited in the le of this patent UNITED STATES PATENTS 1,901,663 Minkler Mar. 14, 1933 2,209,064 Nyquist July 23, 1940 2,214,130 Green et al. Sept. 10, 1940 2,414,107 Kenyon Ian. 14, 1947 2,422,698 Miller June 24, 1947 2,426,454 Johnson Aug. 26, 1947 2,498,103 Wojciechowski Feb. 21, 1950 2,538,789 Maynard Jan. 23, 1951 2,544,685 Jackson Mar. 13, 1951 2,557,186 Hamacher June 19, 1951 

